Keep information about originating backend in ElDoc doc snippets

Keep information about originating backend in ElDoc doc snippets

This lays groundwork for discriminating between different
documentation providers in ElDoc display outlets, i.e. members
of eldoc-display-functions

* lisp/emacs-lisp/eldoc.el (eldoc--make-callback): Take extra origin arg.
(eldoc-documentation-compose-1)
(eldoc-documentation-compose-eagerly)
(eldoc-documentation-default): Pass extra arg to eglot--make-callback.
(eldoc--invoke-strategy): Rework.